During the recovery of hydrocarbons from geological formations, significant amounts of hydrocarbons are left behind because injected or natural drive fluids in the formation are produced along with the oil to such an extent that the cost of fluid disposal makes further oil recovery uneconomical. In formations with high permeability strata, fractures, or wormholes, natural drive fluids (such as brine or gaseous hydrocarbons) in primary recovery processes or flooding fluids (such as brine, stream or carbon dioxide) in secondary recovery processes flow through highly permeable zones, resulting in progressively less hydrocarbon being recovered per unit volume of fluid produced. This increased ratio of drive or flooding fluid to hydrocarbons is usually due either to early breakthrough of flooding fluid from injector wells to producer wells, or to excessive water encroachment into producer wells. It has adversely affected the economics of recovery processes in many parts of the world. For example, it was recently estimated that in the United States, 7 barrels of water are produced for each barrel of oil, amounting to 2.1.times.10.sup.10 barrels of water annually. In Alberta, the ratio of water/oil procduced is 5/1, amounting to 2.6.times.10.sup.9 barrels of water produced in 1995.
The art of controlling or modifying fluid flows in the recovery of hydrocarbons from underground formations is commonly referred to as "conformance control". For the past two decades, research has been directed at improving the oil/water ratio during production by using chemical gel systems to block water flow through high permeability zones, fractures and wormholes (referred to herein as "channels"). The general approach has been to inject a mixture of reagents, initially low in viscosity, into regions of a formation which have high permeability channels. Once the mixture of reagents has reached its destination in the desired region of the formation, it then undergoes a chemical reaction to produce a gel which is capable of blocking the flow of water. Polymers, chemical gels, silica gels, and other blocking agents have been used in this way for conformance control in geological formations.
Ideally, a gel system for conformance control should have the following properties:
1. The reagents should be easily delivered to the desired location in the formation. The components therefore should be initially of low viscosity. No component should be adsorbed out prior to reaching its destination, and each component should be stable to shear stress encountered during delivery. PA1 2. The chemical reaction(s) required for gelation under the conditions found in the formation. PA1 3. The gel generated should be of high strength the conditions found in the formation. PA1 4. The degree of permeability reduction should be high. PA1 5. The system should be of low enough cost to make it economically feasible. PA1 6. The system should have minimal environmental impact. PA1 1. Polyacrylamide copolymers which are injected together with a cross-linking agent, Chromium (III) or Aluminum (II); PA1 2. Xanthan gum (a natural heteropolysaccharide) which together with a cross-linking agent, Chromium (III); PA1 3. Poly (Vinyl alcohol) which is injected together with a cross-linking agent, gluteraldehyde; and PA1 4. Acidified sodium silicate, which when neutralized, rapidly undergoes polymerization to forrn spherical silica particles.
All of the chemical gel systems currently available for conformance control have the drawback of being so costly that their use is limited. Examples of existing gel systems are:
The most widely used method of these involves use of polyacrylamide cross-linked with chromium ions. Its use is limited by its cost: approximately $1000 is required to deliver 1 cubic meter of gel solution into a formation. It is also relatively unstable under the elevated temperature conditions which exist in geological formations during thermal recovery processes. Furthermore, Chromium (VI), the oxidation product of Chromium (III) is highly toxic, so the use of Chromium (III) as a cross-linking agent can be an environmental concern.
There is therefore a need to develop further conformance control gel systems which are environmentally safe, inexpensive and effective under the conditions encountered during hydrocarbon recovery.